The present invention relates to a method for generating a lens position signal, which describes the position of the optical axis of an objective lens of an apparatus for reading from and/or writing to an optical recording medium with regard to the optical axis of an optical scanner used in this apparatus, and also to a correspondingly configured apparatus for reading from and/or writing to an optical recording medium.
A track error signal is conventionally generated in apparatuses for reading from and/or writing to optical recording media, such as, for example, optical recording media (e.g. DVD-RAM) in which information tracks are contained both in depressions (G), designated as “groove”, and in elevations (L), designated as “land”, which track error signal can be used for tracking regulation in the respective apparatus. One of the widespread methods for forming the track error signal is the so-called “differential push-pull” (DPP) method, as is described for example in EP 0 745 982 A2. In this case, the laser beam output by a laser diode is split into three beams, namely a primary beam and two secondary beams which scan mutually adjacent tracks of the optical recording medium respectively used. The primary and secondary beams reflected from the optical recording medium are evaluated in order to obtain, in a manner dependent thereon, primary-beam and secondary-beam track error signals from which the desired track error signal is generated by means of weighted combination.
A corresponding arrangement is illustrated by way of example in FIG. 8. The light emitted by a light source or a laser 1 passes through a collimator lens 2 and is then split into the primary beam (i.e. a 0th-order beam) and the two secondary beams (i.e. ±1st-order beams) by a diffraction grating 3. The primary beam, which reads the information to be scanned in a track of a corresponding recording medium 7, usually contains the majority (approximately 80–90%) of the light information. The two secondary beams each contain the remaining 5–10% of the total light intensity, it being assumed for the sake of simplicity that the light energy of the higher orders of diffraction of the diffraction grating 3 is zero. These three beams are focused onto the optical recording medium 7 via a polarizing beam splitter 4 and a quarter-wave plate 5 and also an objective lens 6, in order to read from and/or write to the said optical recording medium. The three beams reflected from the optical recording medium 7 are fed via the beam splitter 4 and a cylindrical lens 8 to a photodetector unit 9, which detects the three beams reflected from the optical recording medium 7. The three beams are indicated symbolically in the figure between cylindrical lens 8 and photodetector unit 9. Connected to the photodetector unit 9 is an evaluation unit 10, which evaluates the detected signals of the reflected primary and secondary beams for the purpose of generating a track error signal.
The diffraction grating 3 is incorporated in such a way that the imaging of the two secondary beams scans precisely the centre of the secondary tracks or (in the case of media which can be written to only in “groove” tracks) the centre beside the track scanned by the primary beam. Since the secondary beams and the primary beam are intended to be optically separable from one another, the positions of their imaging on the optical recording medium 7 and on the photodetector unit 9 are separate from one another. If the optical recording medium 7 rotates, then one of the secondary beams is situated in front of, and the other secondary beam behind, the primary beam in the reading or writing direction. The evaluation unit 10 of the arrangement shown in FIG. 8 evaluates the light intensities reflected onto the photodetector 9 separately for each of the three beams.
In the evaluation unit 10, the detected signals both of the primary beam and of the secondary beams are used to generate, considered by themselves in each case, a push-pull signal which represents the track error of the respective beam with respect to the track. However, since the two secondary beams scan the secondary tracks with respect to the read/write track, their push-pull track error is inverted with respect to that of the primary beam. Considered by themselves, the respective push-pull components thus contain the actual track error with respect to the respectively scanned track. Since the track position of the three beams can only change together, the three push-pull signals change equally.
The objective lens 6 of an optical scanner 21 as sketched in FIG. 8 must be mounted in a movable manner in order, even in the case of an optical recording medium 7 which has vertical wobble and/or eccentricity, to make it possible to focus the scanning beam and keep it on a predetermined track. That part of the scanner 21 which comprises the elements 2, 3, 4, 5, 8, 9 defines an optical axis 22. The objective lens 6 is arranged in its rest position ideally in such a way that its optical axis 23 corresponds to the optical axis 22 of the other optical components of the optical scanner 21.
The movement of the objective lens 6 is usually achieved by means of an electromagnetic drive. In this case, the objective lens is kept in a predetermined rest position by an arrangement of articulated joints or springs, from which position it can be deflected from its rest position by application of a current to the electromagnetic drive. To that end, the output signals of the evaluation unit 10 provide track error and focus error signals which encompass the position of the objective lens 6 and correct it with the aid of regulating circuits.
If the intention is to scan an optical recording medium 7 whose tracks are applied in spiral form, then the objective lens 6 is deflected to an increasing extent during a continuous scanning operation. Its optical axis 23 is therefore displaced increasingly far from the optical axis 22 of the other optical components. In order to counteract this displacement of the optical axes with respect to one another, provision is usually made of a servo or linear motor which subsequently shifts the scanner 21 with the optical components 2, 3, 4, 5, 8, 9 incorporated therein in such a way that the optical axes deviate from one another as little as possible. This motor is usually referred to as coarse track motor. According to the prior art, the driving voltage of the electromagnetic drive of the objective lens is used as a criterion for the deviations of the optical axes and the coarse track motor is driven in such a way that the driving voltage tends to zero.
For this purpose, provision is made of a further regulating circuit, which ensures that the optical axes 22, 23 of scanner 21 and objective lens 6 correspond. According to the prior art, the driving voltage of the electromagnetic drive of the objective lens 6 is evaluated for this purpose. In this case, it is assumed that the optical axis 23 of the objective lens 6 does not deviate from the axis of the other components 22 when the drive coils are de-energized. Since the objective lens is suspended in a resilient fashion, this assumption is not correct in all operating situations. By way of example, the objective lens changes its position even without driving of the drive coils if external forces act on it, as may occur in the event of an impact against the player. Furthermore, ageing of the articulated joints or springs may cause the rest position of the objective lens to be changed such that the optical axes deviate from one another. These effects cannot be detected from the driving voltage of the drive coils.
If the objective lens 6 is then moved for example during a track jump, the imaging of the primary and secondary beams on the photodetector unit 9 also moves. This displacement of the imaging results in an offset voltage at the output of the evaluation unit 10, the direction of this offset voltage being identical for all of the beams. The displacement of the objective lens 6 thus gives rise to an offset voltage which does not originate from an actual track error and is therefore an interference. The genuine track error component and the undesirable lens-movement-dependent component are added in the push-pull signal which is detected by the respective detectors of the photodetector unit 9 and yielded by the evaluation unit 10.
If the push-pull signals of the secondary beams are then added and this sum is subtracted from the push-pull signal of the primary beam, this undesirable lens-movement-dependence component is cancelled out given appropriate weighting between the primary and secondary beam components. By contrast, since the push-pull components of primary and secondary beams are inverted with respect to one another, they are added in the correct phase after application of the subtraction, with the result that, given correct setting of the weighting factor, the actual track error is obtained. A method for determining a suitable weighting factor is described by way of example in EP 0 708 961 B1.
From the previously described properties of the conventional DPP method, it is apparent that, owing to the position of the secondary beams, the phase shift between the primary beam and the secondary beams is nominally 180 degrees. This is advantageous since, as a result of the difference formation, the track error components of the primary beam and of the secondary beams are added with the largest possible amplitude. If the position of the beams on the tracks is considered, then the angle of the diffraction grating 3 is set, for achieving the maximum amplitude of the track error signal, precisely in such a way that (for example in the case of a DVD-RAM) the secondary beams impinge on the track centres of the secondary tracks or (in the case of media which can be written to only in “groove” tracks) precisely on the region between two tracks beside the track scanned by the primary beam.
The aim of the DPP method described previously is to form a track error signal which has no offset dependence on the position of the objective lens 6 relative to the optical axis of the scanner respectively used. In the case of the previously described combination of the push-pull components of the primary beam and of the secondary beams, although the actual track error can be obtained, owing to the cancellation of the lens-movement-dependence component it is nonetheless not possible to detect the position of the objective lens 6 with regard to the optical axis of the scanner.
During a track following operation, the objective lens 6 is displaced perpendicularly to the track direction of the optical recording medium 7, i.e. the optical axis of the objective lens 6 is moved away from the optical axis of the scanner 21. This results in a corresponding displacement of the imaging of the reflected scanning beam on the detector elements of the photodetector unit 9. While the respectively scanned track is followed correctly, the evaluation unit 10 cannot recognize in this case that the optical axes of objective lens 6 and scanner 21 do not correspond. For this reason, it is necessary, in principle, to provide a signal which describes the position of the objective lens 6 with regard to the optical axis 22 of the scanner 21.
It is furthermore advantageous, during a positioning operation, as is necessary for example for an access to another piece of music on a CD, to provide for the control unit of the apparatus auxiliary signals which enable a fast access to the piece of music desired by the user of the apparatus.